This invention relates generally to heat exchangers, and specifically to a metallic heat exchanger for transferring heat from a heat-emitting medium to a heat-absorbing medium through the walls of a number of flow tubes through which the one medium flows, these tubes being surrounded by the other medium. The heat exchanger is especially intended for use when preheating the combustion air for an oil-fired industrial furnace with its own flue gases.
A major problem with heat exchangers used for the purpose mentioned is the great danger of corrosion of those components of the heat exchanger exposed to high temperatures while simultaneously being in contact with the flue gases from the furnace. At temperatures above approximately 600.degree. C., the material corrodes quite rapidly, mainly as a result of the action of sulfur oxides, vanadium oxides and alkali oxides contained in the fuel oil. In addition, the danger of corrosion is also great when the material temperature drops below approximately 150.degree. C., this corrosion resulting mainly from the precipitation of acidic alkali sulfates on the heat exchanger surfaces. Furthermore, these corrosion products are adherent and lead to clogging of the gas channels.
The indicated problem occurs mainly in heat exchangers of the counterflow type, these providing the best heat utilization with the lowest possible heat transfer area. In heat exchangers of the parallel flow type, the danger of corrosion is low, but the heat utilization in such heat exchangers is limited in that the cold heat-absorbing medium cannot be heated to above approximately 100.degree. C. below the outlet temperature of the hot heat-emitting medium.
To reduce the danger of high-temperature corrosion while simultaneously achieving a heat exchanger with small dimensions and satisfactory heat utilization, the simultaneous use of the parallel flow and counter-flow principles and, if possible, even combining these with the crossflow principle in one and the same heat exchanger is already known. In this case, the outlet for the cold medium is located adjacent to the inlet for the warm medium, where an initial heat exchange section is provided. After passing through this section, the cold medium is fed to a region adjacent to the outlet for the warm medium, where a second heat exchange section, of what is essentially the counter-flow type, is located. Generally, this section extends to the initial section.
Various embodiments of heat exchangers of this type are described, by way or example, in the U.S. Pat. No. 1,673,418 and in the German Pat. No. 1,551,553. The devices described in these publications have in common, complicated elements for generating the desired flow in the medium surrounding the flow tubes, this medium being preferably the cold medium. In addition, this medium flows for long distances without having contact with the walls of the flow tubes, resulting in complicated and expensive devices yielding a rather low heat utilization in relation to their size. Further, only some of the medium surrounding the flow tubes flows past the hottest section of these tubes. As a result when the amount of medium flowing through the heat exchanger is reduced, for example from a reduction in output of the furnace whose flue gases are being fed through the flow tubes of the heat exchanger concerned, the flow conditions for the cold medium become generally unfavorable so that overheating in the first heat transfer section results. In the second heat transfer section the cold medium can attain a higher temperature than intended, leading to high-temperature corrosion, or it can cool the warm medium too much, leding to low-temperature corrosion and a danger of clogging.
It is known further that radial or axial flanges can be attached to the outer surface of the flow tubes to control the flow of the medium surrounding these tubes in such a manner that a combination of the parallel flow and counterflow principles results, the entire flow of the medium surrounding the tubes thus passing through all of the various sections of the heat exchanger. A device of this type, for example, is shown in the German Patent No. 1.050.489, especially in FIG. 6. With such a design, one obtains a relatively compact embodiment of the heat exchanger, but the design is very complicated, especially with regard to the tubes, and assembly requires great precision. Any changes of the flow conditions in the cold medium require complete reconstruction of the entire heat exchanger.